Adding a prefix to the name of that unit forms a multiple of a unit in the International System (see the table "International System Prefixes").The prefixes change the magnitude of the unit by orders of 10 from 1018 to 10-18.

Very small concentrations of chemicals are also measured in parts per million (ppm) or parts per billion (ppb), which mean just what they sound like: If there are four parts per million of lead in a rock (4 ppm), then out of every million atoms in that rock, on average four of them will be lead.

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